1 .\" This manpage is Copyright (C) 1992 Drew Eckhardt;
2 .\" and Copyright (C) 1993 Michael Haardt, Ian Jackson.
3 .\" and Copyright (C) 2004, 2006, 2007, 2014 Michael Kerrisk <mtk.manpages@gmail.com>
5 .\" %%%LICENSE_START(VERBATIM)
6 .\" Permission is granted to make and distribute verbatim copies of this
7 .\" manual provided the copyright notice and this permission notice are
8 .\" preserved on all copies.
10 .\" Permission is granted to copy and distribute modified versions of this
11 .\" manual under the conditions for verbatim copying, provided that the
12 .\" entire resulting derived work is distributed under the terms of a
13 .\" permission notice identical to this one.
15 .\" Since the Linux kernel and libraries are constantly changing, this
16 .\" manual page may be incorrect or out-of-date. The author(s) assume no
17 .\" responsibility for errors or omissions, or for damages resulting from
18 .\" the use of the information contained herein. The author(s) may not
19 .\" have taken the same level of care in the production of this manual,
20 .\" which is licensed free of charge, as they might when working
23 .\" Formatted or processed versions of this manual, if unaccompanied by
24 .\" the source, must acknowledge the copyright and authors of this work.
27 .\" Modified 1993-07-21 Rik Faith (faith@cs.unc.edu)
28 .\" Modified 1994-08-21 by Michael Chastain (mec@shell.portal.com):
29 .\" Removed note about old kernel (pre-1.1.44) using wrong id on path.
30 .\" Modified 1996-03-18 by Martin Schulze (joey@infodrom.north.de):
31 .\" Stated more clearly how it behaves with symbolic links.
32 .\" Added correction due to Nick Duffek (nsd@bbc.com), aeb, 960426
33 .\" Modified 1996-09-07 by Michael Haardt:
34 .\" Restrictions for NFS
35 .\" Modified 1997-09-09 by Joseph S. Myers <jsm28@cam.ac.uk>
36 .\" Modified 1998-01-13 by Michael Haardt:
37 .\" Using access is often insecure
38 .\" Modified 2001-10-16 by aeb
39 .\" Modified 2002-04-23 by Roger Luethi <rl@hellgate.ch>
40 .\" Modified 2004-06-23 by Michael Kerrisk
41 .\" 2007-06-10, mtk, various parts rewritten, and added BUGS section.
43 .TH ACCESS 2 2015-07-23 "Linux" "Linux Programmer's Manual"
45 access, faccessat \- check user's permissions for a file
48 .B #include <unistd.h>
50 .BI "int access(const char *" pathname ", int " mode );
52 .BR "#include <fcntl.h> " "/* Definition of AT_* constants */"
53 .B #include <unistd.h>
55 .BI "int faccessat(int " dirfd ", const char *" pathname ", int " \
56 mode ", int " flags );
60 Feature Test Macro Requirements for glibc (see
61 .BR feature_test_macros (7)):
70 _XOPEN_SOURCE\ >=\ 700 || _POSIX_C_SOURCE\ >=\ 200809L
80 checks whether the calling process can access the file
84 is a symbolic link, it is dereferenced.
88 specifies the accessibility check(s) to be performed,
89 and is either the value
91 .\" F_OK is defined as 0 on every system that I know of.
92 or a mask consisting of the bitwise OR of one or more of
93 .BR R_OK ", " W_OK ", and " X_OK .
95 tests for the existence of the file.
96 .BR R_OK ", " W_OK ", and " X_OK
97 test whether the file exists and grants read, write, and
98 execute permissions, respectively.
100 The check is done using the calling process's
102 UID and GID, rather than the effective IDs as is done when
103 actually attempting an operation (e.g.,
106 Similarly, for the root user, the check uses the set of
107 permitted capabilities rather than the set of effective
108 capabilities; and for non-root users, the check uses an empty set
111 This allows set-user-ID programs and capability-endowed programs
112 to easily determine the invoking user's authority.
115 does not answer the "can I read/write/execute this file?" question.
116 It answers a slightly different question:
117 "(assuming I'm a setuid binary) can
118 .I the user who invoked me
119 read/write/execute this file?",
120 which gives set-user-ID programs the possibility to
121 prevent malicious users from causing them to read files
122 which users shouldn't be able to read.
124 If the calling process is privileged (i.e., its real UID is zero),
127 check is successful for a regular file if execute permission
128 is enabled for any of the file owner, group, or other.
132 system call operates in exactly the same way as
134 except for the differences described here.
136 If the pathname given in
138 is relative, then it is interpreted relative to the directory
139 referred to by the file descriptor
141 (rather than relative to the current working directory of
142 the calling process, as is done by
144 for a relative pathname).
154 is interpreted relative to the current working
155 directory of the calling process (like
165 is constructed by ORing together zero or more of the following values:
168 Perform access checks using the effective user and group IDs.
171 uses the real IDs (like
174 .B AT_SYMLINK_NOFOLLOW
177 is a symbolic link, do not dereference it:
178 instead return information about the link itself.
182 for an explanation of the need for
185 On success (all requested permissions granted, or
189 and the file exists), zero is returned.
190 On error (at least one bit in
192 asked for a permission that is denied, or
196 and the file does not exist, or some other error occurred),
199 is set appropriately.
207 The requested access would be denied to the file, or search permission
208 is denied for one of the directories in the path prefix of
211 .BR path_resolution (7).)
214 Too many symbolic links were encountered in resolving
224 does not exist or is a dangling symbolic link.
227 A component used as a directory in
229 is not, in fact, a directory.
232 Write permission was requested for a file on a read-only filesystem.
241 points outside your accessible address space.
245 was incorrectly specified.
248 An I/O error occurred.
251 Insufficient kernel memory was available.
254 Write access was requested to an executable which is being
257 The following additional errors can occur for
262 is not a valid file descriptor.
265 Invalid flag specified in
272 is a file descriptor referring to a file other than a directory.
275 was added to Linux in kernel 2.6.16;
276 library support was added to glibc in version 2.4.
279 SVr4, 4.3BSD, POSIX.1-2001, POSIX.1-2008.
286 Using these calls to check if a user is authorized to, for example,
287 open a file before actually doing so using
289 creates a security hole, because the user might exploit the short time
290 interval between checking and opening the file to manipulate it.
291 .BR "For this reason, the use of this system call should be avoided" .
292 (In the example just described,
293 a safer alternative would be to temporarily switch the process's
294 effective user ID to the real ID and then call
298 always dereferences symbolic links.
299 If you need to check the permissions on a symbolic link, use
302 .BR AT_SYMLINK_NOFOLLOW .
304 These calls return an error if any of the access types in
306 is denied, even if some of the other access types in
310 If the calling process has appropriate privileges (i.e., is superuser),
311 POSIX.1-2001 permits an implementation to indicate success for an
313 check even if none of the execute file permission bits are set.
314 .\" HPU-UX 11 and Tru64 5.1 do this.
315 Linux does not do this.
317 A file is accessible only if the permissions on each of the
318 directories in the path prefix of
320 grant search (i.e., execute) access.
321 If any directory is inaccessible, then the
323 call will fail, regardless of the permissions on the file itself.
325 Only access bits are checked, not the file type or contents.
326 Therefore, if a directory is found to be writable,
327 it probably means that files can be created in the directory,
328 and not that the directory can be written as a file.
329 Similarly, a DOS file may be found to be "executable," but the
331 call will still fail.
334 may not work correctly on NFSv2 filesystems with UID mapping enabled,
335 because UID mapping is done on the server and hidden from the client,
336 which checks permissions. (NFS versions 3 and higher perform the check on
338 Similar problems can occur to FUSE mounts.
341 .SS C library/kernel differences
344 system call takes only the first three arguments.
348 .B AT_SYMLINK_NOFOLLOW
349 flags are actually implemented within the glibc wrapper function for
351 If either of these flags is specified, then the wrapper function employs
353 to determine access permissions.
355 On older kernels where
357 is unavailable (and when the
360 .B AT_SYMLINK_NOFOLLOW
361 flags are not specified),
362 the glibc wrapper function falls back to the use of
366 is a relative pathname,
367 glibc constructs a pathname based on the symbolic link in
369 that corresponds to the
373 In kernel 2.4 (and earlier) there is some strangeness in the handling of
376 If all categories of execute permission are disabled
377 for a nondirectory file, then the only
379 test that returns \-1 is when
391 returns 0 for such files.
392 .\" This behavior appears to have been an implementation accident.
393 Early 2.6 kernels (up to and including 2.6.3)
394 also behaved in the same way as kernel 2.4.
396 In kernels before 2.6.20,
397 these calls ignored the effect of the
399 flag if it was used to
401 the underlying filesystem.
402 Since kernel 2.6.20, the
414 .BR path_resolution (7),
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